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Weather Bee | How unusual is the heavy rain flooding northeastern states

Jul 04, 2024 08:00 AM IST

India’s northeastern region has been affected by floods and landslides this monsoon season. How much of a role does rain play in these disasters?

Almost all the eight states in India’s northeastern region have been affected by floods and landslides this monsoon season. While both these disasters can be a function of multiple factors, how much is rain playing a role? This is important to ask because even small amounts of rain can flood an area if it is not planned for rain or if there is an unusually large discharge of water upstream of a river. Similarly, landslides can impact mountainous and hilly regions — as parts of all northeastern states are — due to mining or unplanned construction. While the role of these factors cannot be discounted, an HT analysis of the India Meteorological Department’s (IMD) gridded data shows that all northeastern states are indeed experiencing an unusually wet monsoon season.

People wade through flood waters on a boat at Lehpati village of Morigaon district in India's northeastern state of Assam on July 3, 2024. (Photo by Biju BORO / AFP)(AFP) PREMIUM
People wade through flood waters on a boat at Lehpati village of Morigaon district in India's northeastern state of Assam on July 3, 2024. (Photo by Biju BORO / AFP)(AFP)

The trend in rain is usually checked against the average of rain over a long time period, called the Long Period Average (LPA). The IMD currently uses the 1971-2020 period for calculating the LPA. An analysis of 2024 monsoon rain (the season officially begins on June 1) shows that four of the eight northeastern states — Meghalaya, Sikkim, Assam, and Nagaland — have more than 20% surplus over the LPA, which is IMD’s threshold for “excess” or “large excess” rain. The rain for the season so far is also ranked very high for Meghalaya, Sikkim, and Assam: fifth, ninth, and 12th highest for this period since 1901.

Departure of 2024 rain from LPA: June 1 to July 3(Abhishek Jha)
Departure of 2024 rain from LPA: June 1 to July 3(Abhishek Jha)

To be sure, the other four states not breaking records for the season overall does not mean that they are dry. The past week has been exceptionally rainy in all northeastern states except Sikkim. The rain in 2024 for the week ending July 3 is among the top ten highest recorded since 1901 in all these states except Sikkim, where it is the 40th highest. Last week’s rain is between 1.5 and 2.3 times the LPA in the seven states, excluding Sikkim.

Departure of 2024 rain from LPA: June 27 to July 3(Abhishek Jha)
Departure of 2024 rain from LPA: June 27 to July 3(Abhishek Jha)

How much of these states have been affected by excess or large excess rain? In all states except Manipur, where overall rain in the season is not ranked high, at least 25% of the state’s area is affected by excess or large excess rain this monsoon. In Sikkim, which has been affected by floods and landslides since the beginning of the season, 92% of the area is affected by such a large surplus of rain. Similarly, in Assam, Meghalaya, and Nagaland, at least half of the state’s area is affected by excess or large excess rain.

Distribution of state's area by performance of monsoon(Abhishek Jha)
Distribution of state's area by performance of monsoon(Abhishek Jha)

IMD’s data also shows that parts of all northeastern states have been affected by more than usual days of heavy or extreme rain (rain of more than 35.5 mm in 24 hours). Over 50% of all northeastern states, except Manipur, have experienced more days of heavy or extreme rain than in the LPA. In Sikkim, Meghalaya, Assam, and Nagaland, such area covers 80%-100% of the state’s area.

Proportion of state's area where heavy-extreme rain days increased compared to LPA: June 1 to July 3(Abhishek Jha)
Proportion of state's area where heavy-extreme rain days increased compared to LPA: June 1 to July 3(Abhishek Jha)

The data described above makes it clear that all northeastern states are experiencing an exceptionally wet monsoon season so far, although in Manipur this trend might have started slightly later than in other states. However, the loss of lives and property is almost always a function of the resilience we build to extreme weather events. Such resilience can be built either through the management of the river’s flow or its banks, early warning systems and effective communication of adverse weather events, protecting the natural environment from exploitation, and other measures that are well-documented by the scientific community. The lack of action on these measures causes loss of lives and property in extreme weather events.

Abhishek Jha, HT’s assistant editor-data, analyses one big weather trend in the context of the ongoing climate crisis every week, using weather data from ground and satellite observations spanning decades.

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